This invention relates to a solid electrolytic capacitor and more particularly to an improved cathode especially for a small solid electrolytic capacitor body. Numerous means are known for forming a cathode electrode and attaching a cathode lead wire thereto. The typical solid electrolytic capacitor has a porous valve-metal anode pellet, usually cylindrical, with one anode lead attached; a dielectric film is formed over the surfaces of the pellet and this is covered by a solid electrolyte; next a layer of graphite is applied; then the graphite layer is metallized and a cathode lead wire is attached to the metallized layer.
One known method for attaching the cathode lead wire is to butt solder it, another consists in placing an end of the cylindrical wire flush with the surface of the metallized surface and applying solder, and yet another method employs a lead wire that is spirally wrapped around the graphite coated body and is attached thereto by an overcoat of solder.
Of the first two methods for attaching the cathode lead wire to a small pellet (e.g. a pellet having a diameter less than about five times that of the cathode lead wire to be attached), the solder connection tends to crack and the solder joint between lead and pellet consequently exhibits a high resistance during subsequent handling such as in treating, testing, and/or encapsulating. This typically shows up as a high dissipation factor or open circuit and usually is cause for rejecting the part. The last mentioned means for connecting a cathode lead wire to a pellet is less prone to crack, however all three methods of cathode lead attachment are difficult to implement especially when employing a small pellet since accurate registration of the wire and the pellet prior to soldering cannot be accomplished reliably and surely on a ganged or multiple capacitor basis as in a high speed large scale manufacturing operation. The first two mentioned cathode lead structures are therefore produced with less than desirable final yields and the yield diminishes as the size of the pellet becomes smaller. The latter mentioned method of spiralling the wire about the body can only be accomplished one at a time and tends to produce a capacitor of large diameter.
It is therefore an object of the present invention to provide a solid electrolytic capacitor having a strong low resistance connection between the capacitor body and the cathode lead wire.
It is a further object of this invention to provide a solid electrolyte capacitor capable of being produced with high yields.
It is yet a further object of this invention to provide an electrolytic capacitor employing a small anode and including a reliable cathode structure that contributes minimally to the diameter or bulk of the finished capacitor package.